Problem: In physics class, the girl to boy ratio is $3$ to $8$. If there are a total of $33$ students, how many girls are there?
A ratio of $3$ girls to $8$ boys means that a set of $11$ students will have $3$ girls and $8$ boys. A class of $33$ students has $3$ sets of $11$ students. Because we know that there are $3$ girls in each set of $11$ students, the class must have $3$ groups of $3$ girls each. There is a total of $9$ girls in physics class.